Self-drilling screws have been known for a considerable period of time, and have undergone a substantial amount of development. In spite of that development, it remains necessary for such screws to have different characteristics according to some circumstances of use. In particular, screws which are intended to be used with metal plate, that is, relatively thick material, will have different characteristics from those intended for use with sheet metal, that is relatively thin material.
A screw intended for use in sheet metal will normally have a relatively deep thread because a relatively low torque will be required for the screw to form a complimentary thread, that is, the tapped thread, in the hole formed by the drilling tip. It is difficult however, for the screw to form the same depth of thread in metal plate, and in some cases the torque required is excessive. That has led to the development of two classes of screws within the same diameter range. The screws in each class have a thread of the same crest and root diameters, but the drill tip diameter is different in each case.
A relatively large drill tip diameter is required for the class of screw intended for use in metal plate, as that reduces the depth of the tapped thread and therefore reduces the torque required to produce that thread. The same screw is not satisfactory for use in sheet metal because there is insufficient depth in the tapped thread. In the case of sheet metal, there needs to be a substantial amount of engaging thread surface between the screw and the sheet metal in order to impart sufficient resistance to stripping of the tapped thread. If the depth of the tapped thread is relatively small, a relatively low torque applied to the screw during the tightening operation may be sufficient to strip that thread.
The difficulty which flows from the foregoing is that users of such screw products need to carry both classes of such screw products, and need to make the correct selection according to the use of the particular screw in any particular circumstances. Selection can be a problem unless there is a clearly defined separation between the two classes of screw products, because the only difference between screws of the same thread diameter is the diameter of the drill tip and that may not be a clearly visible difference in many cases.